Computer devices and other integrated circuit devices may need to transmit messages to or receive messages from other devices. In some cases the message transmission may occur between two interconnected devices or between a large number of devices forming a network. In such a network one or more of the devices may be in the form of computer apparatus and other devices may comprises a variety of peripheral equipment.
The transmission of data between the integrated circuit devices can be accomplished by a variety of different techniques. For communication over short distances it is common to use a parallel data bus with many signal wires shared between many devices. For communication over longer distances a serial communication system may be preferred with pairs of devices connected by one or two signal wires. The more devices that a shared bus connects, the slower the maximum speed of operation. This imposed a practical limit on the number of devices which may be connected by a shared data bus. Furthermore, it is inconvenient to use many parallel buses in a system on account of the large number of signal lines and difficulties of arranging for communication between devices attached to different buses. There is no limit to the number of devices which may be connected using individual serial connections between pairs of devices provided that each device is equipped with a number of links to which connections can be made. However, existing serial connection systems are too slow, too big or inappropriate in function to be used for general purpose communications between microcomputers and other devices in a system.